1. Field of the Invention
This invention relates generally to the enhancement of biological nitrification processes for the removal of ammonia nitrogen from wastewater, and more particularly to the protection of such processes from ammonia toxicity.
2. Information Disclosure Statement
Nitrification is an oxidative process for converting ammonia nitrogen to nitrite and nitrate. The reactions are commonly carried out biologically to remove nitrogen from wastewater before disposal or reuse.
Organisms responsible for oxidation of carbonaceous organic material are ubiquitous and generally heterotrophic organisms such as zooglea, pseudomonas and chromobacterium which require organic carbon as a food and energy source.
On the other hand, organisms responsible for nitrification are classed as chemotrophic because of their ability to fix inorganic carbon (CO.sub.2) as their carbon source. Nitrosomonas and Nitrobacter represent the limited groups of microorganisms responsible for nitrification, and which obtain energy from oxidation of ammonia nitrogen to nitrite and nitrate by the following two-step pathway:
Step 1: NH.sub.4.sup.+ +1.5 O.sub.2 .fwdarw.NO.sub.2.sup.- +2 H.sup.+ +H.sub.2 O (Nitrosomonas) PA1 Step 2: NO.sub.2.sup.- +0.5 O.sub.2 .fwdarw.NO.sub.3.sup.- (Nitrobacter)
Nitrification may be carried out in suspended growth systems or attached growth treatment schemes. Nitrification may be accomplished in one or more separate stages provided for that purpose, or nitrification can be performed together with organic carbon removal in the same vessel or vessels. Although considerable nitrification is accomplished in well-operated activated sludge systems treating municipal wastewaters, stringent removal requirements often require a much higher degree of nitrogen removal than is achieved with conventional systems for organic carbon removal.
All biological treatment systems, including nitrification systems, are susceptible to upsets caused by sudden changes in temperature, pH, dissolved oxygen concentration or the presence of toxic substances. Death of part of the population leaves the same amount of substrate that must be removed by a smaller number or organisms. The remaining organisms must reproduce to replace the organisms lost in the upset. Therefore, system recovery is dependent on the specific growth rate (reproduction rate) of the microorganisms. Nitrifying bacteria are strictly autotrophic, and have a much slower specific growth rate than heterotrophic bacteria responsible for degradation of organic material. The slow growth rate decreases the ability of the nitrifiers to recover from system shocks, such as pH or temperature excursions, low dissolved oxygen (DO) levels, or the presence of toxic materials. Ammonia nitrogen (particularly as free ammonia) is itself inhibitory or toxic to nitrification bacteria. When a shock of any kind disables the nitrifiers, the ammonia nitrogen in the wastewater often rises to levels which are inhibitory or toxic. These elevated ammonia nitrogen levels hinder the recovery of the nitrifying cultures; high removal rates of nitrogen may not be achieved for weeks or months following an upset.
Ammonia nitrogen is thought to inhibit nitrifier growth or activity at concentrations as low as 100 to 200 mg/l. The severity of inhibition increases as the ammonia nitrogen concentration increases. Domestic wastewaters typically contain 10 to 50 mg/l of ammonia nitrogen. During upset conditions, ammonia released by lysing biomass may increase the level dramatically. Many industrial wastewaters contain considerably higher ammonia nitrogen concentrations, as high as several g/l or higher. Start-up of nitrification systems to treat such wastewaters may be protracted for several months or longer. Thus, the overall nitrogen removal efficiency is severely hampered.
In suspended growth systems, biological upsets are often accompanied by washout of a substantial portion of the microorganisms from the system due to difficulties in solids separation. Heterotrophic bacteria recover much more rapidly than chemotrophic nitrifiers due to their higher specific growth rate.
A small portion of wastewater nitrogen is assimilated into new cellular material. Typically, viable activated sludge contains 6-14 percent nitrogen as percent of solids. The actual quantity of nitrogen removed from the wastewater into cellular material depends on the quantity of biological solids removed (wasted) from the system. Where the solids residence time in the system is relatively long, the fraction of ammonia nitrogen removed by cell assimilation is very small.
Generally, conditions which cause an upset of the nitrifying bacteria often upset the carbon-consuming heterotrophic microorganisms as well. Typically, when such an upset occurs, steps are taken to prevent solids washout, in order to maintain as high a population of microorganisms as possible under the adverse conditions. A negative effect of this procedure is that a sizeable fraction of the bacterial population, deactivated by the shock conditions, lyse and release additional ammonia nitrogen into the wastewater through enhanced endogenous respiration. Thus, a step taken to restore the carbon-consuming bacteria to a "healthy" condition contributes to the toxicity or inhibitory effects of the wastewater on the nitrifying organisms.
Systems where both carbonaceous material and ammonia are simultaneously removed in the same oxygenated vessel are exemplified by the activated sludge process. The conditions for optimal removal rates of the two types of contaminant differ, however, so that generally the system is designed and controlled for removal of carbonaceous matter, with nitrogen removal being a secondary consideration. Ammonia nitrogen removal is generally low, often less than 35 percent, and large fluctuations are typical.
A two stage biological system employing a carbon removal step followed by an ammonia removal step is shown in Stankewich, Jr. U.S. Pat. No. 3,764,523. A different biological sludge is maintained within each stage by separation and circulation of each sludge. Pure oxygen is used to maintain relatively high concentrations of dissolved oxygen in both steps. A small quantity of wastewater carbonaceous BOD is allowed to enter the second stage to provide sufficient carbonaceous material for production of nitrifying organisms.
Laughton U.S. Pat. No. 4,160,724 shows a wastewater treatment system with two biological steps using the same sludge comprising a mixture of carbon consuming organisms, nitrifying organisms and denitrifying organisms. The first step is controlled as an anoxic stage for denitrification and partial removal of carbonaceous matter, and the second stage is controlled at a higher dissolved oxygen concentration to enhance nitrification. A high recycle rate ensures that the nitrified wastewater from the second stage is subjected to denitrifying conditions.
A similar flowsheet is shown in Klapwijk et al U.S. Pat. No. 4,183,809, with the additional feature whereby a portion of the wastewater bypasses the first (denitrification) stage and is routed to the second (nitrification) stage before passing to the first stage.
The two-stage nitrification-denitrification process of Knopp et al U.S. Pat. No. 3,957,632 uses powdered activated carbon in admixture with biomass in both stages. The benefits included maintenance of a high concentration of nitrifying organisms in the first (nitrification) stage and a high concentration of denitrifying organisms in the second (denitrification) stage.
Simultaneous nitrification and denitrification in a single stage is shown in Wong-Chong U.S. Pat. No. 4,537,682. The invention not only includes control of sludge wastage rate, hydraulic residence time, dissolved oxygen requirement, sludge mixing rate, pH and temperature, but also the control of the influent BOD.sub.5 /N ratio at a level of at least 1.7-2.8 to satisfy denitrification requirements. From a practical standpoint, such control is extremely difficult to achieve.